Polyhaloaromatics such as polychlorinated biphenyls are organic chemicals that have been widely produced at the beginning of the 20th century. These compounds possess excellent heat stability, are nonflammable in nature, have low volatility and good viscosity characteristics at operating temperatures. These properties made certain polyhaloaromatics ideal for use as dielectric materials for transformers and capacitors. These compounds have also been used as efficient heat transfer agents.
However, extensive studies on halogenated biphenyls have shown that the excellent stability of these products made their presence in the environment hazardous and the compounds have been positively identified as possible carcinogens. Accordingly, a number of countries such as the United States now prohibit manufacture, importation or use of these compounds on their territory. Furthermore, extensive programs to remove and destroy most of the compounds produced so far have been established.
PCBs have been mainly used in electrical insulating oils contained in electrical transformers, capacitors, underground cables and the like. This was the most important use for these materials that have in many instances been replaced with different non-hazardous materials. Hence, the need to find a safe and efficient method to reduce PCBs to environmentally safe compounds became quite obvious. In that respect, the scientific literature provides an extremely broad array of processes aimed at dehalogenating organic halides.
So far, it seems that the most economically feasible process for dehalogenating PCBs is their conversion into non-toxic compounds through incineration at high temperatures, usually above 1000.degree. C. Obviously, this combustion must be carried out in specially designed high temperature furnaces. Also, the combustion process yields the formation of gases that must be vented into the atmosphere. However, if the combustion is incomplete or interrupted, extremely toxic compounds such as polychlorinated dibenzofurans may be produced.
A large number of prior art documents describe processes for dehalogenating organic halides at moderate temperatures ranging between 80.degree. and 200.degree. C. which involve contacting an organic halide solution with small particles of alkali metals. Such processes are described in the following references: E.P No. 0099951, Journal of Hazardous Materials, 12, 1985, 161-176 (A. Cornel et al), U.S. Pat. No. 4,379,746, U.S Pat. No. 4,340,471, U.S. Pat. No. 4,377,471, U.S. Pat. No. 4,416,767, Japanese Patent No. 49082570, Can. Patent No. 1,185,265 and Can. Patent No. 1,179,381.
In these references, whether the alkali metal is used alone or in conjunction with another compound, complete dehalogenation is rarely obtained and the processes often lead to polymerization. It is also worth mentioning that although in some instances processes using an alkali metal either alone or with other substances are successful in dehalogenating organic halides, the usual temperature at which the process must be performed generally remains quite above room temperature.
Furthermore, most methods that were developed to degrade PCBs in alkali metal media involve the use of coreagents that are expensive, sensitive to impurities and hazardous to the environment. For example, degradation of PCBs in silicon-based oil, benzophenone, alkylbiphenyls, naphthalene or anthracene, requires careful separation of these chemicals at the end of the reaction before any safe disposal into the environment can be foreseen.
In a recently published article, K.M. Anwer et al (J. Org. Chem., 54, 1284-1289, 1989) describe a reaction through which chloroaromatics are dechlorinated using ammonium formate on a supported Pd/C catalyst. This reaction requires the use of expensive catalysts that are sensitive to impurities. It is also worth mentioning that dechlorination is influenced by the solvent. For example, dechlorination of 1-Cl-naphthalene was only 51% complete in tetrahydrofurans (THF).
Finally, organic reductions by alkali metals in neat ammonia require special equipment and careful precautions to handle ammonia gas. Furthermore, when organic halides, particularly aromatic halides, are reduced by sodium and neat ammonia, they often produce toxic aromatic amines in a complex pool of other products.
Therefore, it would be highly desirable to provide a process efficient to successfully dehalogenate organic halides using inexpensive reagents while producing environmentally safe by-products.